1. Field of the Invention
The invention concerns a detection and locating apparatus and method using an optical fiber or similar waveguide as a distributed sensor by which a disturbance can be sensed and its location determined to a point along the path of the optical fiber. According to an inventive aspect, polarization effects managed to make such detection dependable notwithstanding changing conditions, and to provide a robust ratio of signal to noise.
In one embodiment, two light signals are developed and coupled into two counter-propagating light signal channels carried by at least one waveguide. The counter-propagating light signals are locally affected by substantially the same physical disturbance, for example, an increase in pressure or a vibration or the like, that may deform the fiber and at least affects the optical propagation conditions locally. A resulting change is detected in the two counter-propagating light signals, with a temporal shift resulting from the light signals having propagated along different paths of potentially different length. From the temporal shift, a processor calculates the location along the path at which the disturbance affected the counter-propagating light signals.
The waveguide can be an optical fiber or two or more optical fibers or plural modes in a given fiber, in each case supporting propagation of a beam in the waveguide. The opposite light signals can originate from different light sources and/or can be subdivided beams from a same source, such as subdivided portions of a coherent laser beam. Although propagating in opposite directions, the two light signals are affected by the disturbance in substantially the same way but propagate over distances and times that are independent of one another.
Physical disturbances such as pressure or stress from moving masses and other events of potential security interest cause polarization altering changes in both of the counter-propagating optical signals. Such changes are detectable according to the invention with sensitivity and precision. The optical fiber waveguide medium is insensitive to electromagnetic interference, intrinsically safe, stable and reliable. However, at the scale of the wavelength of the light signals, momentary stresses and the like produce variations that are readily detectable as phase variations leading to a change in polarization states.
Although the disclosed technology can be applied to various position sensing situations, this disclosure uses the example of optical fiber based perimeter security as a non-limiting example. Inasmuch as an optical waveguide is easily placed to follow various paths, the same technique can be used to extend a detection path between arbitrary zones, to provide a two or three dimensional detection area, etc.
2. Prior Art
A security system should detect and provide information about any intrusion into a protected area or facility. An advantageous system should discreetly detect even modest physical disturbances, and report the location of the disturbance so as to permit corrective action to ensue promptly. If a security system is not visible or otherwise apparent to an intruder, it is more difficult for the intruder to proceed undetected than if elements of the security system are not concealed. There may be a deterrence benefit, however, in making it known that a facility is equipped with security devices.
Some optical sensors rely on gross effects of an intruder's presence, such as the intruder interrupting a beam that is aimed from a source to a sensor. Other sensors rely on proximity or the like. Whether the effect is gross or subtle, there is a need to know not only that a disturbance has occurred but also to know where the disturbance occurred. With one signal path, it may be possible from changes in the received signal to determine that a disturbance has occurred, but not to know where. One technique for localizing a disturbance is by determining the difference in timing between the appearance of effects of a disturbance, in two signals that are both affected by the disturbance. A relative delay in appearance of the disturbance in a signal propagating on one path versus another path, indicates a longer propagation distance from the disturbance to the detector where the signal is detected. If there are two or more operative paths, measuring the delay can permit one to calculate an apparent location of the disturbance. This technique is described in British Patent GB 1,497,995—Ramsay, entitled “Fiber Optic Acoustic Monitoring Arrangement.”
Optical fiber has inherent advantages, such as low loss, immunity to electromagnetic interference and other characteristics, that are useful in remote sensing. Optical fiber interference sensors as in Ramsay have the additional advantages of geometric versatility (i.e., the fiber can follow almost any desired route), wide dynamic range, and high sensitivity, partly due to the very short wavelength of the electromagnetic radiation (light energy) that is carried in an optical fiber. The measurement of the delay in Ramsay and other similar detectors is the phase difference between light from a given source, received over two different paths, such as counter-propagating paths, of potentially different length. The phase difference is detected at the receiving end of both paths, by causing the light from the two fibers to interfere, i.e., to add constructively or destructively at a summing node. As the signals move in and out of phase, the intensity of the interference sum varies between a maximum and a minimum.
An example of an interference sensor is the Mach-Zehnder interferometer, which has been applied to acoustic sensing, magnetic sensing, temperature sensing, pressure sensing, structure monitoring, etc, including using optical fibers, as described in “Overview of Mach-Zehnder sensor Technology and Applications” by Anthony Dandridge and Alan D. Kersey, Fiber Optic and Laser Sensors VI, Proc. SPIE Vol. 985, pp. 34–52 (1988).
In addition to GB 1,497,995—Ramsay, cited above, the publication “Fiber Optic Distributed Sensor in Mach-Zehnder Interferometer Configuration” by Bogdan Kizlik, TCSET'2002 Lviv-Slavsko, Ukraine proposes location fixing techniques. Recent US Patents and publications including U.S. Pat. No. 6,621,947 and US 2003198425 teach the possibility of a perimeter defense system based on the same principle.
These prior art teachings rely on interference of periodic waves to produce a variation in intensity that reflects the parameter that is needed to determine a location from a difference in propagation time over two distinct signal paths. For example, the disturbance may induce a change in propagation conditions such as a local change in optical index. Such a change effectively shortens or elongates the optical path. Where two beams traverse the optical path, both beams are affected. The effect of the disturbance on either one of the two signals is a phase shift, reaching the detector after that signal has propagated from the disturbance to the detector. An interference summing node is effectively a phase comparator. According to the theory, for a time between arrival at the detector of the first phase shifted signal along one path, and arrival of the second phase shifted signal along a different path, a short term phase difference persists between the two received signals. As a result, if the interfering signals produced a given intensity amplitude due to the constructive or destructive positioning of their phases, that amplitude changes upon arrival of the first signal and returns to its previous level upon arrival of the second signal. The time span is a function of the difference in distances from the detector to the disturbance along the two paths. From the time difference and information as to whether the phase difference leads or lags, the disturbance can be located to a point. {It will till work even if it is in the middle, just means that the time difference is zero.}
There is a problem, however, in applying the theory. Light energy is not a simple planar wave, and optical waveguides induce polarization effects. When attempting to use optical fiber waveguides and the like for location detection in this way, polarization effects and polarization induced phase delays can defeat the ability of an interferometer to produce a robust and dependable signal.
Light waves have mutually orthogonal polarization vector components that can be relatively large or small in comparison to one another, and can vary in their phase relationship. For light waves to interfere, there must be an extent of correspondence in their polarization states. Two light waves that are orthogonally polarized cannot interfere. Over plural paths between a light source and two or more detectors, each passing a point of disturbance, the birefringence of different paths and fibers can change the polarization alignment of a light signal. Birefringence changes polarization alignment by inducing a phase difference between two orthogonal components of a light signal. Thus, the change in polarization alignment can involve a phase difference of its own. Although an optical fiber may have small birefringence as compared to its refractive index, an accumulated polarization effect arises, particularly over a long distance, and the effect can be large on the scale of wavelength. An interferometer-based system cannot perform consistently, and in some circumstances will not perform at all, if polarization effects cause the polarization states of the counter-propagating optical signals that are intended to interfere actually to vary between states wherein the constituent components of the signals are more or less parallel and more or less orthogonal at different times.
Adverse effects on interfering beams due to polarization state changes over a single light path is known as polarization-induced fading. The problem is described, for example, in “Polarization-induced Fading in Fiber-Optic Sensor Arrays” (Moshe Tur, Yuval S. Boger, and H. J. Shaw, Journal of Lightwave Technology, Vol. 13, No. 7, p 1269, 1995). This publication seeks to enhance the visibility of the interference beam in a single-channel fiber based interferometer, where the light travels along a single direction.
Polarization induced phase shift is a somewhat different effect from polarization induced fading, but causes measurement problems because polarization induced phase shift can be difficult to distinguish from other factors. If there is a polarization induced phase shift, the effective phase shifts may not correlate well between the two counter propagating signals received at the detector. The technique of calculating a location for the disturbance relies on identifying corresponding phase shifts in each of the two counter-propagating signals and measuring the lead/lag time between their appearance at the detector. Such a measurement is not possible if variations in the two signals cannot be matched.
Polarization phase shift variations arise in part because there are dynamically varying changes to the polarization states of the light signals between the signals as they are launched, versus the principal polarization axis of the interferometer at which the received signals can potentially interfere constructively or destructively. The difference varies as a function of the birefringent state of the fiber along the two counter-propagation paths. If the states of polarization of the two interfering beams are not parallel to each other, then the intensity response due to the interference will be affected according to the alignment or misalignment of the vector components of the two beams. The polarization state of a light signal involves not only the angular alignment of its orthogonal components but also a phase relationship at a given point along a propagation axis. Dynamic changes along the propagation paths induce phase factors depending on the mismatch of polarization alignments; and the depth or span of potential intensity modulation due to interference is reduced (signal fading).
This polarization dependent effect, which can be termed the polarization induced phase shift, depends on the polarization mismatch. The change in polarization state between the point of launch and the point of detection (interference) generally is not the same for the two counter-propagating light signals. However, a temporal difference between corresponding phase changes in the counter-propagating light signals is to be the parameter used for localizing the disturbance. The unequal additional polarization induced phase shift results in errors in determining the correct location.
An interferometer produces an intensity response by causing phase varying signals to add or to cancel at different phase positions (i.e., to interfere), and as a result, the effect of polarization fading and polarization induced phase shift can be quite detrimental, leading to system failure if precautions are not taken. Occasional or uncontrollable system failure is unacceptable for a system deployed for security purposes. GB 1,497,995—Ramsay (supra) and other known fiber based perimeter security systems as described, detect variations in intensity from interfering two beams and are subject to fading and phase shift with changes in polarization of beams passed through a fiber interferometer in opposite directions. This limits effectiveness of such systems.